It's not impossible to grow lettuce throughout the summer, but it does take a little extra thought.
Choose the Right Lettuce Variety
First, choose leaf varieties rather than head forming lettuces. You can start harvesting leaf lettuces as soon as the outer leaves reach about 4-6 inches in height. If you cut just these outer leaves, it allows the remaining center leaves to continue growing. This is called "cut and come again".
Not only to you get to start harvesting early, cutting like this tends to shock the lettuce plant, preventing it from thinking it has matured and is ready to bolt and go to seed.
Provide Some Shade
Secondly, plant your lettuce in the shade of taller plants, like tomatoes, corn, or even vining crops like cucumbers and squash. You can do this when you first start seeding, in the spring, or when there are bare spots in the garden to fill. Although lettuce needs more sun in the cool spring than it does in summer, positioning lettuce plants around taller plants, like tomatoes, will provide full sun in spring while the tomatoes are still short, but will offer relief from the intense summer sun.
Keep Your Lettuce Plants Well Watered
Thirdly, regular watering makes plants very forgiving. Water your lettuce plants everyday, more if it is extremely hot and dry. The leaves, which are mostly water, will desiccate and wilt in strong sunlight and dry soil.
Lettuce roots tend to be shallow, so it's more important to water often, than to water deeply.
Shock Them into Growing
If all else fails and it looks like your lettuce plants are ready to bolt, dig them out of the ground and replant them. As with 'cut and come again', this is a shock to the plant's system and your lettuce seedling will once again focus on growing roots and put off setting seed.
Don't keep them out of the ground or allow them to dry out. Just the act of lifting them is enough of a shock.
How to Start Lettuce Seeds in Summer
To have lettuce plants to harvest throughout the summer, you will probably have to start seed when the weather is warm. Lettuce seed can be difficult to sprout, in warm, dry conditions. Try this trick to get them going.
- Find a somewhat shady spot in your garden and give is a deep soaking of water. Then lay a board over the damp soil. The board should be at least the size of the area you intend to seed with lettuce.
- Lift the board and re-soak the soil for another 2 -3 days. This should lower the temperature of the soil.
- Now, plant your lettuce seeds and water them in well. Replace the board, on to of the seeded ground.
- Lift and water every day, replacing the board, until you see signs of germination. It should only take about a 7 - 10 days for the lettuce seeds to sprout. At the first sign of green, remove the board. Keep watering, whenever the soil dries out. This can mean watering more than once a day, while the seedlings are tiny.
Once the plants are a few inches tall and ready to start harvesting, they should not need a great deal of additional water.